Json.Net：HTML助手方法无法重新生成

Question

我遇到一个问题，即我创建的ASP.NET MVC html帮助器方法在每次调用时都不会“重新生成”。

helper方法的目的是创建要在angularjs框架中使用的Javascript对象。例如，这是一个使用helper方法的代码片段（从html页面的script标签内调用）：

var app = angular.module( "appName", ["ui.bootstrap"] );

app.controller( 'appCtrl', function( $scope ) {
    $scope.model = @Html.ToJavascript( Model, new string[] { "FirstName", "LastName", "ID", "Role" } );
} );

Model是具有各种属性的类的实例，但我只希望将FirstName，LastName，ID和Role序列化为javascript对象。

Statis类中定义了ToJavascript（）帮助器方法，如下所示：

   public static HtmlString ToJavascript( this HtmlHelper helper, object toConvert, string[] includedFields = null, Formatting formatting = Formatting.Indented, ReferenceLoopHandling loopHandling = ReferenceLoopHandling.Ignore )
    {
        using( var stringWriter = new StringWriter() )
        using( var jsonWriter = new JsonTextWriter( stringWriter ) )
        {
            var serializer = new JsonSerializer()
            {
                // Let's use camelCasing as is common practice in JavaScript
                ContractResolver = new SpecificFieldsResolver( includedFields ),
                Formatting = formatting,
                ReferenceLoopHandling = loopHandling,
            };

            // We don't want quotes around object names
            jsonWriter.QuoteName = false;
            serializer.Serialize( jsonWriter, toConvert );

            return new HtmlString( stringWriter.ToString() );
        }
    }

这利用Json.NET进行实际的序列化。

Json.NET的许多很酷的功能之一是它允许您即时定义要序列化的字段。那就是SpecificFieldsResolver所做的。我将其定义如下：

public class SpecificFieldsResolver : CamelCasePropertyNamesContractResolver
{
    private string[] _included;

    public SpecificFieldsResolver( string[] included )
    {
        _included = included;
    }

    protected override JsonProperty CreateProperty( MemberInfo member, MemberSerialization memberSerialization )
    {
        JsonProperty prop = base.CreateProperty( member, memberSerialization );

        bool inclField = ( _included == null )
            || _included.Contains( member.Name, StringComparer.CurrentCultureIgnoreCase );

        prop.ShouldSerialize = obj => inclField;

        return prop;
    }
}

让我感到困惑的是CreateProperty（）的调用方式。具体来说，对于要序列化的每种类型的对象，似乎只调用一次。

这是一个问题，因为在另一个cshtml文件中，我再次调用ToJavascript（），它试图序列化相同类型的对象，但是要从序列化中输出不同的字段：

var app = angular.module( "app2Name", ["ui.bootstrap"] );

app.controller( 'app2Ctrl', function( $scope ) {
    $scope.model = @Html.ToJavascript( Model, new string[] { "FirstName", "LastName", "ID", "Role", "Category", "VoterID" } );
} );

Category和VoterID也是有效的类字段。但是ToJavascript（）不会将它们序列化。相反，它只会序列化在第一次调用ToJavascript（）中定义的字段…即使该调用发生在另一个cshtml文件中。就像SpecificFieldsResolver记住它创建的JsonProperty对象一样。

有什么想法吗？

更新资料

感谢dbc诊断确切的错误并提出解决方法。之所以略加适应，是因为我在几个解析器中依赖于Json.NET的驼峰案例名称解析：

public class CamelCaseNameMapper : CamelCasePropertyNamesContractResolver
{
    public string ToCamelCase( string propertyName )
    {
        return ResolvePropertyName( propertyName );
    }
}

public class MaoDefaultContractResolver : DefaultContractResolver
{
    private CamelCaseNameMapper _mapper = new CamelCaseNameMapper();

    protected override string ResolvePropertyName( string propertyName )
    {
        return _mapper.ToCamelCase( propertyName );
    }

}

现在，从MaoDefaultContractResolver派生的每个解析器（例如我的SpecificFieldsResolver）都会自动继承驼峰式大小写，但避免了dbc标识的缓存问题。

Answer 1

这似乎是的错误CamelCasePropertyNamesContractResolver。它的基类DefaultContractResolver拥有两个构造函数：无参数构造函数和DefaultContractResolver (Boolean)版本（在Json.NET
7.0中已作废）。该参数含义如下：

shareCache

• 类型：System.Boolean

如果设置为true，DefaultContractResolver将使用与相同类型的其他解析程序共享的缓存。共享缓存将显着提高多个解析器实例的性能，因为昂贵的反射将仅发生一次。如果假定解析程序的不同实例产生不同的结果，则此设置可能导致意外的行为。设置为false时，强烈建议使用重复使用DefaultContractResolver实例JsonSerializer。

默认值为false。

不幸的是，默认构造函数用于CamelCasePropertyNamesContractResolver将值设置为
true ：

public class CamelCasePropertyNamesContractResolver : DefaultContractResolver
{
    public CamelCasePropertyNamesContractResolver()
#pragma warning disable 612,618
        : base(true)
#pragma warning restore 612,618
    {
        NamingStrategy = new CamelCaseNamingStrategy
        {
            ProcessDictionaryKeys = true,
            OverrideSpecifiedNames = true
        };
    }
}

此外，没有第二个带有该shareCache选项的构造函数。这打断了你的SpecificFieldsResolver。

解决方法是，您可以从解析器派生DefaultContractResolver并用于CamelCaseNamingStrategy进行名称映射：

public class IndependentCamelCasePropertyNamesContractResolver : DefaultContractResolver
{
    public IndependentCamelCasePropertyNamesContractResolver()
        : base()
    {
        NamingStrategy = new CamelCaseNamingStrategy
        {
            ProcessDictionaryKeys = true,
            OverrideSpecifiedNames = true
        };
    }    
}

public class SpecificFieldsResolver : IndependentCamelCasePropertyNamesContractResolver
{
    // Remainder unchanged
}

请注意，如果您使用的是9.0之前的Json.NET版本，CamelCaseNamingStrategy则不存在。而是CamelCasePropertyNamesContractResolver可以使用嵌套的kludge
来映射名称：

public class IndependentCamelCasePropertyNamesContractResolver : DefaultContractResolver
{
    class CamelCaseNameMapper : CamelCasePropertyNamesContractResolver
    {
        // Purely to make the protected method public.
        public string ToCamelCase(string propertyName)
        {
            return ResolvePropertyName(propertyName);
        }
    }
    readonly CamelCaseNameMapper nameMapper = new CamelCaseNameMapper();

    protected override string ResolvePropertyName(string propertyName)
    {
        return nameMapper.ToCamelCase(propertyName);
    }
}